A high efficiency motor, which can utilize magnet torque as well as reluctance torque by burying interior magnets within a rotor core, is well known in the market, one example is disclosed in the Japanese Patent Application Unexamined Publication No. H08-331823. FIG. 18 is a cross section illustrating this kind of conventional motor.
In FIG. 18, stator 401 comprises a plurality of teeth 403 and yoke 405 which connects roots of the plural teeth, and stator 401 is shaped as a ring. A plurality of slots 407 formed between teeth 403 are wound in a three-phase winding manner.
Rotor 410 is substantially coaxial with stator 401 and is shaped substantially as a cylinder, and faces an inner wall of stator 401 via an annular space. Rotor 410 has four rotor poles, and is supported by a bearing (not shown) so that rotor 410 can rotate on shaft 421. Four slits 413 bored through axially and disposed at equal intervals along a rotational direction of rotor core 411 are provided on rotor 410, and plate-like permanent magnet 415 is inserted into each slit. A terminal plate (not shown) is disposed on each axial end of rotor core 411 to secure permanent magnet 415. The terminal plate is fixed on the end face by riveting pin 425 through hole 423, whereby the permanent magnet 415 is fixed into rotor core 411. An outer wall of rotor 410 has notches 419 at boundary areas between the respective rotor poles, and both of longitudinal ends of permanent magnet 415 are adjacent to respective notches 419. An electric current runs through the stator coil to create a rotating magnetic field. Then, the rotor poles attract/repel teeth 403 of stator 401, thereby spinning rotor 410.
In the above structure, the following relation is established between inductances Ld and Lq: Ld&lt;Lq
where Ld is an inductance along "d" axis which crosses the rotor pole at right angles, and PA1 Lq is an inductance along "q" axis extending through the boundary area between the rotor poles. PA1 .psi.a: interlinkage magnetic flux PA1 I: stator coil current PA1 .beta.: leading phase angle of the current I (electrical angle) PA1 (a) a stator core having a plurality of teeth and a yoke connecting the teeth; PA1 (b) concentrated windings on the teeth; PA1 (c) a rotor having interior permanent magnets; PA1 and the rotor with the interior permanent magnets comprises the following elements: PA1 (a) stator core having a plurality of teeth and a yoke connecting the teeth; PA1 (b) windings on the teeth; PA1 (c) a rotor having interior permanent magnets; PA1 and the rotor comprises the following elements: PA1 (a) stator core having a plurality of teeth and a yoke connecting the teeth; PA1 (b) windings on the teeth; PA1 (c) a rotor having interior permanent magnets; PA1 and the rotor comprises the following elements: PA1 (a) stator core having a plurality of teeth and a yoke connecting the teeth; PA1 (b) concentrated windings on the teeth; PA1 (c) a motor including a rotor with interior permanent magnets; PA1 and the rotor with the interior permanent magnets comprises the following elements:
In general, motor torque "T" is expressed by the following equation: EQU T=Pn{.psi.a.cndot.I.cndot.cos.beta.+0.5(Lq-Ld)I.sup.2.cndot.sin2.beta.}
where, Pn: a number of paired rotor poles,
In the equation discussed above, the first term represents magnet torque, and the second represents reluctance torque. Since the relation of Ld&lt;Lq is satisfied, a winding current is so controlled to advance the phase of the winding current "I" with regard to respective induced voltage generated in each phase winding, thereby .beta. becomes greater than zero (.beta.&gt;0), and the reluctance torque is generated. When .beta. is set at a predetermined value, the greater torque "T" can be produced with a same current than the case where only the magnet torque is available.
However, according to the above structure, since steel section 417 having high permeability exists between slit 413 and notch 419, magnetic flux at the end of permanent magnet 415 runs through magnetic path 430 of steel section 417. The magnetic flux is thus short-circuited as shown in FIG. 18, although it should have reached stator 401 and contributed to torque production. In other words, the magnetic flux decreases by the short-circuited amount, thereby lowering a motor efficiency. Further, the magnetic flux resulted from short-circuited increases cogging torque, which increases noises and vibrations of the motor.
A motor employing another type of interior permanent magnets is disclosed in the Japanese Patent Application Unexamined Publication No. H08-331783 as shown in FIG. 19. This prior art, different from the conventional motor discussed above, has no notches on the outer wall, and a distributed winding method is employed on stators.
In FIG. 19, rotor 503 has four sets of interior permanent magnets 501 and 502 in rotor core 507 made of electromagnetic stacked steel sheets. Magnets 501 and 502 are placed on each pole in a radial direction with a space. Each set of magnets 501 and 502 are places such that "S" pole and "N" pole are adjacent to each other. Magnets 501 and 502 forming a layer structure are placed such that the outer sides of respective outer magnets 501 facing the outer rim of rotor 503 have the same polarity as the outer sides of respective inner magnets 502. Magnets 501 and 502 are shaped as arcs and show their hills toward the rotor center, and the two magnets in the layer structure form substantially concentric circles and lie in parallel. A space between the two magnets is substantially constant.
Rotor 503 as defined above is rotated by composite torque of magnet torque and reluctance torque: i.e. the magnet torque is generated by the relation between the magnet field of magnets 501 and 502 and rotating magnetic field produced by the current running through a group of coils 509 striding over a predetermined number of teeth 506 defined by broken lines 517, on the other hand, the reluctance torque is generated by magnetic paths formed by the rotating magnetic field and appearing between magnets 501 and 502 as well as on the surface of rotor 503. The reluctance torque is thus utilized, thereby realizing a higher efficiency motor than the motor using only magnet torque.
Among such motors as employing interior permanent magnets, another idea is carried out not only to increase efficiency but also to downsize the motor, i.e. a concentrated winding method is practiced on the teeth of stator in a higher density.
However, in conventional motor with the concentrated winding on the teeth, an adjacent magnetic pole turns to an opposite pole when the motor is powered with three-phase 120.degree. current and winding volume for one pole and one phase is concentrated on one tooth. Magnetizing force is thus as strong as twice of the motor with the distributed winding, and therefore magnetic flux runs between the adjacent teeth, which demagnetizes the interior permanent magnets.